In general, force-transmitting devices are used to transmit a force from a mechanical power source to a driven member. An example of a typical driven member can be, among others, a drill bit, a saw blade, a pump, or an automotive road wheel. An example of a typical power source can be, among others, an electric motor or an internal combustion engine.
Typically, an engine or motor produces mechanical power in the form of a rotational or torsional force. In such a case, the force-transmitting device used to transmit the rotational force is usually in the form of a shaft which rotates about a longitudinal rotational axis as it transmits the power between the engine or motor and the driven member.
In an automotive environment, for example, the power source is usually in the form of a gasoline or diesel engine which is mounted on an automotive vehicle. In such a case, the driven member is one or more driven road wheels that are attached to the vehicle. At least one force-transmitting device is usually used to transmit the rotational power produced by the engine to the driven road wheel, or wheels, for the purpose of propelling the vehicle. The engine and the driven road wheels, together the force-transmitting device and associated components, are usually referred to as the "drive train" of the vehicle. Two common examples of force-transmitting devices that are included in vehicle drive trains are those of drive shafts and axle shafts.
Generally, a rotating shaft type of force-transmitting device, such as that used in an automotive drive train, has a central shaft portion and a coupling portion mounted on each of the two ends of the shaft portion. The coupling portions are usually configured to facilitate the coupling of the force-transmitting member into force-transmitting relation with other components of the drive train. The shaft portion is usually configured to efficiently transmit rotational power between the two coupling portions.
The coupling portions can each have a number of different configurations depending on the specific application of the force-transmitting device. For example, in an automotive environment, a coupling portion can be configured, among others, as a component of: a universal joint, a constant velocity joint, a mounting plate, a spline, a gear, and a road wheel mounting hub.
Likewise, the shaft portion can have a number of different configurations depending on the specific application. The most common configurations of the shaft portion are those of a hollow tube, and that of a solid bar. As such, the tube or bar may have any number of different shapes, but the most prevalent shape is that of an elongated circular cylinder.
One consideration in the overall configuration of a force-transmitting device is the weight, or mass, of the device. The overall mass of a force-transmitting device can be an important consideration in some applications. For example, the mass of a force-transmitting device can effect the fuel efficiency of the automotive vehicle in which the device is installed. A force-transmitting device can effect the fuel efficiency of a vehicle from both a gravitational standpoint and an inertial standpoint.
From a gravitational standpoint, the more mass a vehicle has, the more energy the vehicle engine must produce to overcome gravitational forces. For example, a vehicle made of heavier material will result in more rolling resistance at the tire/road surface contact point than a similar vehicle which is made from material that weighs less. Similarly, a vehicle made from relatively heavy material will require more energy to propel the vehicle up a hill than a vehicle made from relatively light material.
From an inertial standpoint, generally, a vehicle made of heavier material will require more energy to accelerate at a given rate than a vehicle made of relatively light material. Similarly, a rotating component of an automobile drive train, such as a force-transmitting device which is made from a relatively heavy material will require more energy to rotationally accelerate at a given rate than a similar component of lighter material. Further, by reducing rotational mass, a rotating shaft can be accelerated or slowed more rapidly than a heavier shaft. This can be beneficial for example in performance automobiles where it is desirable to accelerate or decelerate a rotating shaft, for example, when changing speeds when entering a corner.
There are also other instances in which the weight of the material used in making a force transmitting device is an important consideration. For example, another application of a force transmitting device is that of drill string, or drill rod, used in geological drilling or boring operations. In certain instances, drill string sections must be transported long distances to and from the drilling site, and can even be transported by helicopter. The use of a relatively lighter material in the manufacture of drill string sections would thus result in lower costs associated with transporting such drill string.
Several different types of materials are available for the manufacture of force-transmitting devices. Two popular materials are those of steel and aluminum alloy. Steel has several attributes as a material for force-transmitting devices. One such attribute is that it is relatively inexpensive. Another attribute of steel is its durability. For example, some types of steel can be treated to obtain a high level of hardness relative to other materials. High hardness can be very desirable in certain applications such as, for example, components that are subjected to concentrated stress loads and components subjected to surface-to-surface contact with other components.
However, one of the drawbacks of using steel as a material for force-transmitting devices is that steel has a relatively low strength-to-weight ratio when compared with other suitable materials such as, for example, aluminum alloy. In other words, steel has more mass than other materials at a given strength level. For example, consider two versions of an automobile drive train component such as, for example, a drive shaft. Furthermore, consider the case in which one of the drive shafts is made from steel and the other is made from an aluminum alloy and that both drive shafts have the same force-transmitting capacity. In such a case, the aluminum alloy drive shaft will weigh about half that of the steel drive shaft. Thus, a chief attribute of using aluminum alloy as a material for force-transmitting devices is that aluminum has a relatively high strength-to-weight ratio compared with steel. On the other hand, however, the surface hardness of steel generally is not attainable in aluminum alloy material.
As described above, the use of aluminum alloy as a material for force-transmitting devices would be desirable in cases where weight savings would be beneficial. At the same time, however, the use of aluminum alloy would not be desirable in cases where a high level of hardness is desired or required in the device. Thus, combining aluminum alloy portions with steel portions to produce a "hybrid" force-transmitting device would result in maximum benefits compared with similar devices made entirely from either one or the other material.
Producing such a hybrid force-transmitting device of both aluminum alloy and steel portions, however, would require joining steel and aluminum alloy together. The traditional method of joining steel and aluminum alloy has been that of welding, bolting. Although welding has long been successfully used to join two similar or identical materials, the process of welding dissimilar metals, such as aluminum alloy and steel, has not been perfected to a desirable degree. Specifically, the strength of an aluminum to steel weld is generally relatively low and the result is an unacceptably high rate of weld failure. When dissimilar metals are joined by bolting, the process is time consuming and requires additional parts. In modern assembly processes, such as building of automobiles, it is desirable to reduce the time of assembly as well as the number of components required in the final assembled product.
Therefore, it would be desirable to provide a force-transmitting device which achieves the benefits to be derived from similar prior art devices, but which avoids the shortcomings and detriments individually associated therewith. Specifically, it is desirable to produce an assembly of two metals, one heavier than the other, which will achieve weight savings in the final assembly, and will also reduce the time and cost of joining the two metals.